2 Answers
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The Exif:ApertureValue is stored as an APEX value as mandated by various EXIF standards.

The APEX system is a way to calculate exposure and works using base-2 logarithms. The use of base-2 means a rise of one in the value equates to a doubling, which we know as 1 stop; which makes it pretty handy for photographers if they're good with logarithms (which we don't use much nowadays). Do note, however, that 1 stop in aperture means doubling of the square of the F-number -- for example, going from f/2.8 to f/4 is 1 stop because 42 = 2 * 2.82 (approximately).

This is a partial answer. According to this listExif.Photo.ApertureValue is "The lens aperture. The unit is the APEX value." Which is confirmed by Wikipedia article on APEX. No idea how to make the conversion though.